Not Applicable.
Not Applicable.
During the drilling and completion of oil and gas wells, it is often necessary to engage in ancillary operations, such as monitoring the operability of equipment used during the drilling process or evaluating the production capabilities of formations intersected by the wellbore. For example, after a well or well interval has been drilled, zones of interest are often tested to determine various formation properties such as permeability, fluid type, fluid quality, formation pressure, and formation pressure gradient. Formation fluid samples are also taken for analysis of their hydrocarbon content. These tests determine whether commercial exploitation of the intersected formations is viable.
Formation testing tools are used to acquire a sample of fluid from a subterranean formation. This sample of fluid can then be analyzed to determine important information regarding the formation and the formation fluid contained within, such as pressure, permeability, and composition. The acquisition of accurate data from the wellbore is critical to the optimization of hydrocarbon wells. This wellbore data can be used to determine the location and quality of hydrocarbon reserves, whether the reserves can be produced through the wellbore, and for well control during drilling operations.
Formation testing tools may be used in conjunction with wireline logging operations or as a component of a logging-while-drilling (LWD) or measurement-while-drilling (MWD) package. In wireline logging operations, the drill string is removed from the wellbore and measurement tools are lowered into the wellbore using a heavy cable (wireline) that includes wires for providing power and control from the surface. In LWD and MWD operations, the measurement tools are integrated into the drill string and are ordinarily powered by batteries and controlled by either on-board or remote control systems.
To understand the mechanics of formation testing, it is important to first understand how hydrocarbons are stored in subterranean formations. Hydrocarbons are not typically located in large underground pools, but are instead found within very small holes, or pores, within certain types of rock. The ability of a formation to allow hydrocarbons to move between the pores, and consequently into a wellbore, is known as permeability. Similarly, the hydrocarbons contained within these formations are usually under pressure and it is important to determine the magnitude of that pressure in order to safely and efficiently produce the well.
During drilling operations, a wellbore is typically filled with a drilling fluid (“mud”), such as water, or a water-based or oil-based mud. The density of the drilling fluid can be increased by adding special solids that are suspended in the mud. Increasing the density of the drilling fluid increases the hydrostatic pressure that helps maintain the integrity of the wellbore and prevents unwanted formation fluids from entering the wellbore. The drilling fluid is continuously circulated during drilling operations. Over time, as some of the liquid portion of the mud flows into the formation, solids in the mud are deposited on the inner wall of the wellbore to form a mudcake.
The mudcake acts as a membrane between the wellbore, which is filled with drilling fluid, and the hydrocarbon formation. The mudcake also limits the migration of drilling fluids from the area of high hydrostatic pressure in the wellbore to the relatively low-pressure formation. Mudcakes typically range from about 0.25 to 0.5 inch thick, and polymeric mudcakes are often about 0.1 inch thick. The thickness of a mudcake is generally dependent on the time the borehole is exposed to drilling fluid. Thus, in MWD and LWD applications, where a section of the borehole may be very recently drilled, the mudcake may be thinner than in wireline applications.
Formation testing tools generally comprise an elongated tubular body divided into several tubular modules serving predetermined functions. A typical tool may have a hydraulic power module that converts electrical into hydraulic power; a telemetry module that provides electrical and data communication between the modules and an uphole control unit; one or more probe modules collecting samples of the formation fluids; a flow control module regulating the flow of formation and other fluids in and out of the tool; and a sample collection module that may contain various size chambers for storage of the collected fluid samples. The various modules of a tool can be arranged differently depending on the specific testing application, and may further include special testing modules, such as NMR measurement equipment. In certain applications the tool may be attached to a drill bit for logging-while-drilling (LWD) or measurement-while drilling (MWD) purposes. Examples of such multifunctional modular formation testing tools are described in U.S. Pat. Nos. 5,934,374; 5,826,662; 5,741,962; 4,936,139, and 4,860,581, the contents of which are hereby incorporated herein by reference for all purposes.
In formation testing equipment suitable for integration with a drill string during drilling operations, various devices or systems are provided for isolating a formation from the remainder of the wellbore, drawing fluid from the formation, and measuring physical properties of the fluid and the formation. However, MWD formation testing equipment is subject to harsh conditions in the wellbore during the drilling process that can damage and degrade the formation testing equipment before and during the testing process. These harsh conditions include vibration and torque from the drill bit, exposure to drilling mud, drilled cuttings, and formation fluids, hydraulic forces of the circulating drilling mud, and scraping of the formation testing equipment against the sides of the wellbore. Sensitive electronics and sensors must be robust enough to withstand the pressures and temperatures, and especially the extreme vibration and shock conditions of the drilling environment, yet maintain accuracy, repeatability, and reliability.
In one aspect of formation testing, the formation testing apparatus may include a probe assembly for engaging the borehole wall and acquiring formation fluid samples. The probe assembly may include an isolation pad to engage the borehole wall, or any mudcake accumulated thereon. The isolation pad seals against the mudcake and around a hollow probe, which places an internal cavity in fluid communication with the formation. This creates a fluid pathway that allows formation fluid to flow between the formation and the formation tester while isolated from the wellbore fluid.
In order to acquire a useful sample, the probe must stay isolated from the relative high pressure of the wellbore fluid. Therefore, the integrity of the seal that is formed by the isolation pad is critical to the performance of the tool. If the wellbore fluid is allowed to leak into the collected formation fluids, a non-representative sample will be obtained and the test will have to be repeated.
Examples of isolation pads and probes used in wireline formation testers include Halliburton's DT, SFTT, SFT4, and RDT. Isolation pads that are used with wireline formation testers are generally simple rubber pads affixed to the end of the extending sample probe. The rubber is normally affixed to a metallic plate that provides support to the rubber as well as a connection to the probe. These rubber pads are often molded to fit with the specific diameter hole in which they will be operating. These types of isolator pads are commonly molded to have a contacting surface that is cylindrical or spherical.
While conventional rubber pads are reasonably effective in some wireline operations, when a formation tester is used in a MWD or LWD application, they have not performed as desired. Failure of conventional rubber pads has also been a concern in wireline applications that may require the performance of a large number of formation pressure tests during a single run into the wellbore, especially in wells having particularly harsh operating conditions. In a MWD or LWD environment, the formation tester is integrated into the drill string and is thus subjected to the harsh downhole environment for a much longer period than in a wireline testing application. In addition, during drilling, the formation tester may be constantly rotated with the drill string and may contact the side of the wellbore and damage any exposed isolator pads. The pads may also be damaged during drilling by the drill cuttings that are being circulated through the wellbore by the drilling fluid.
The structure and operation of a generic formation tester are best explained by referring to
In order to acquire a useful sample, probe 112 must stay isolated from the relative high pressure of wellbore fluid 104. Therefore, the integrity of the seal that is formed by isolation pad 110 is critical to the performance of the tool. If wellbore fluid 104 is allowed to leak into the collected formation fluids, an non-representative sample will be obtained and the test will have to be repeated.
For a more detailed description of the embodiments, reference will now be made to the following accompanying drawings:
The drawings and the description below disclose specific embodiments of the present invention with the understanding that the embodiments are to be considered an exemplification of the principles of the invention, and are not intended to limit the invention to that illustrated and described. Further, it is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results.
Various embodiments described provide for isolator pad assemblies especially suited for use in MWD or LWD applications but these assemblies may also be used in wireline logging or other applications. Reference is made to using the embodiments with a formation testing tool, but the embodiments may also find use in any tool that seeks to acquire a sample of formation fluid that is substantially free of wellbore fluid. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results.
Referring to
Referring now to
Referring now to
The extendable test probe assembly 14 is disposed within a corresponding recess 11 in the body 12. The outer surface of the cylinder 17 is in sealing engagement with the inner surface of the cavity in the tool body 12. Thus, the extendable test probe assembly 14 is sealed to and slidable relative to the tool body 12. The extendable test probe assembly 14 also comprises an axial central bore 32 through the cylinder 17. The central bore 32 is in fluid communication with the sample conduit 30.
As shown in
The drilling equipment drills the wellbore 20 until the desired formation 22 to be tested is reached. Drilling operations are then ceased to test the formation 22. The formation tester 10 operates by first extending the extendable test probe assembly 14 by applying fluid pressure through the hydraulic conduit 28 so that hydraulic pressure is applied between the extendable test probe assembly 14 and the body 12. The pressure advances the seal pad 16 toward the wall of the wellbore 20. The seal pad 16 is advanced through the mudcake 24 until the expandable material 40 contacts the formation 22. As the seal pad 16 extends, the expandable material 40 compresses against the formation 22, forming a seal.
As the expandable material compresses against the formation 22, at least a portion of the expandable material 40 expands. The expansion occurs generally in the lateral direction relative to the direction of extension of the extendable test probe assembly 14, but may also occur in other directions. As the expandable material 40 expands, the retainer 44 controls the expansion of the expandable material 40 around the perimeter of the expandable material 40. In the embodiment shown in
As shown in
Once the extendable test probe assembly 14 is in its extended position and a seal formed against the wall of the borehole 20, a sample of formation fluid can be acquired by drawing in formation fluid through the bore 19 of the expandable material and base plate and into the axial central bore 32 of the cylinder 17. As shown in
Referring now to
While specific embodiments have been shown and described, modifications can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments as described are exemplary only and are not limiting. Many variations and modifications are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.